This application claims the benefit of French National Patent Application No. 0116713, filed Dec. 21, 2001, which is incorporated herein by reference in its entirety.
The present invention relates in general to the fabrication of semiconductor substrates, and more particularly, to assembling a donor wafer.
Known techniques for preparing a wafer that includes a thin semiconductor layer for forming circuitry (e.g., electronic, optoelectronic, or optical circuits or components) include Smart-Cut(copyright) type processes. In general, such processes involve implantation of a gaseous species at a controlled depth in a bulk donor wafer in order to create a weakness at a desired depth in the donor wafer, and the application of stresses to cause a separation at the desired depth due to the weakness. Molecular adhesion or wafer bonding may have been used before separation to bond a receiving wafer with a layer of the bulk donor wafer to be separated. Molecular adhesion or wafer bonding may be typical techniques by which the separated layer from the donor wafer and the receiving wafer are assembled. Once separated, further processing for producing circuits or components for the circuits in the separated layer may take place.
In some circumstances (e.g., when re-use is desired), it may be desired to subject the remaining structure of the donor wafer to further processing. The remaining structure may be the subject of mechanical, chemical-mechanical, or other polishing steps to ready remaining portions of the donating material of the donor wafer for further use. Other processing activities may involve chemical cleaning steps, relatively high temperature operations (e.g., 300 to 900xc2x0 C., such as for oxide deposition), or substantially high temperature operations (e.g., 1150xc2x0 C., such as for thermal oxidation in cases such as a silicon carbide wafer).
In some circumstances, it may be desired to recycle the bulk donor wafer through reuse. In such circumstances, the remaining structure may be required to be subject to additional implantation of one or more gaseous species, bonding with a receiving wafer, or further separation steps (e.g., through thermal or mechanical stresses).
Such reuse may progressively decrease the thickness of the donor wafer through consecutive removal of thin layers from the donor wafer. Progressively decreasing the thickness of the donor wafer may lead to an excessively thin donor wafer, which may not be reusable for further recycling.
There are other difficulties or deficiencies that are faced in recycling a donor wafer. There may be a high risk of fracture during predominantly mechanical operations such as when stress is applied to separate a thin layer from the donor wafer or such as when bonding is performed through CMP planarization of a surface oxide, etc. A high risk of fracture also arises for example during high-temperature heat treatment. The risk may be due to non-uniform temperatures in a donor wafer.
There may also be a high risk of fracture when an operator or processing machinery is required to handle a donor wafer. Another deficiency may involve large strains that are induced in certain operating steps when a donor wafer has been thinned through reuse. Operations such as implanting gaseous species or certain deposition steps may induce strains in thinner donor wafers that may cause the wafers to sag significantly (e.g., causing a wafer to take on a convex profile). Sagging may seriously compromise operations that require suitably flat contacting surfaces. Thus, a donor wafer may not be usable for further recycling once a minimum donor wafer thickness has been reached (e.g., a thickness at which deficiencies or drawbacks mentioned above may exist).
Discontinuing recycling at a minimum donor wafer thickness may be economically detrimental and/or inefficient in material consumption because the remaining material is typically discarded as waste material. Deficiency in this process is heightened in cases where the semiconductor material of a donor wafer is relatively expensive (e.g., is a high quality semiconductor material) or relatively fragile. For example, in the case of a standard silicon carbide wafer (e.g., a silicon carbide wafer having a standard diameter of 2 inches), a wafer thinned to about 200 xcexcm may become unusable either because of frequent fractures during the process or because of a sag caused by implantation of gases prevents the wafer from suitably bonding to a receiving wafer.
In other applications, thickness may be relatively thin from a starting donor wafer (e.g., because wafers for a particular semiconductor material are typically offered on the market at that thickness). Gallium Nitride donor wafer may be one such example. Known techniques for producing such wafers involve using a thick eptixay technique called HPVE (Hybrid Vapour Phase Epitaxy) on an epitaxially grown substrate (seed layer) that is removed after epitaxy. This technique, however, has two major drawbacks. Firstly, it only makes it possible to obtain self-supporting wafers having a thickness of at most around 200 to 300 xcexcm. If a greater thickness is sought, imperfect lattice matching with the seed layer may generate excessive strains. Secondly, the rate of growth using the thick epitaxy technique is extremely slow (typically, 10 to 100 xcexcm per hour). Such drawbacks may seriously handicap manufacturing costs.
Drawbacks may also be associated with conventional techniques in which ingots of some single crystal semiconductor material such as SiC are used for producing bulk donor wafers. In conventional techniques in which ingots of semiconductor material such as SiC are used for producing bulk donor wafers, the following operating steps are typically implemented: the ingot may be cut (e.g., using a saw) into slices having a thickness of around 1 mm, each of the faces of the slice may be coarsely polished to remove crystal damaged by sawing and to obtain good planarity, and the future front face (the removal face) may be successively polished to obtain appropriate surface roughness. Such techniques, which may start from relatively thick slices, may often involve substantial losses of material during the successive polishing steps. This obviously affects the manufacturing cost.
Thus, there is a need for providing such processes and structures in a more economically advantageous and efficient way. Within this context, there may also be a need to continue recycling even when extremely small thickness is reached.
In accordance with the principles of the present invention, a process for repeated treatment of wafers may provided that involves preparing a reusable donor wafer for donating a thin layer of semiconductor material by assembling a donor layer of semiconductor material (e.g., a monocrystalline semiconductor material) having a thickness of plural thin layers onto a support layer (e.g., a support layer of a non-monocrystalline semiconductor material). Thus, a support layer is provided in a donor wafer that is of a lower quality or less precious than the donor layer of the wafer. With respect to xe2x80x9cless precious,xe2x80x9d for example, monocrystalline silicon may be considered to be less precious than monocrystalline silicon carbide or another example may be that monocrystalline material of one semiconductor may be considered to be less precious than a higher quality monocrystalline material of the same semiconductor due to differences (e.g., substantial differences) in price, availability, or usability. Such processes may be for providing an electronic, opotoelectronic, or optical component.
In one aspect, a process may be provided for transferring successive thin layers from a semiconductor material of a donor wafer to a receiving wafer. A bulk slice may be assembled that includes a donor layer of a semiconductor material and a support layer. The donor layer and the support layer may form a mechanically stable assembly, which may constitute a donor wafer. A region of weakness may be created in the donor layer at a controlled depth. The donor wafer may be bonded to a receiving wafer via the free side of the donor layer of the donor wafer. A separation may be effected in the region of weakness of the donor layer to transfer a thin layer of the semiconductor material from the donor wafer to the receiving wafer. The process may be repeated to recycle the xe2x80x9cassembledxe2x80x9d donor wafer without breaching the support layer of the donor wafer.
If desired, assembly of the donor wafer may be carried out by wafer bonding using polished faces of the donor layer (which may be a bulk slice) and a support (which may be the support layer). High temperature welding between polished faces may also be used for preparing the assembly. If desired, a region of weakness may be created by implanting gaseous species. In some embodiments, wafer bonding may be implemented to bond the donor wafer to the receiving wafer. Separation of the thin layer may be effected by applying stresses, especially thermal and/or mechanical stresses.
With the use of a support layer, the donor wafer may be recycled a maximum number of times to separate thin layers from the donor layer. The maximum number of times may depend on the thickness of the donor layer and the depth at which a weakness is created in the donor wafer in each cycle.
If desired, the donor layer may be a single crystal semiconductor material and the support may be a single crystal of inferior quality, a single crystal material of a different semiconductor, the same semiconductor in polycrystalline form, or the same semiconductor as a different polytype. The semiconductor material of the donor layer, support layer, or both may for example be silicon, silicon carbide, or large-gap monometallic or polymetallic nitrides. In some embodiments, the donor layer may for example have a thickness of around 100 to 300 xcexcm. In some embodiments, the support layer may for example have a thickness of around 100 to 300 xcexcm.
The semiconductor material of the donor layer, support layer, or both may be a large-gap monometallic or polymetallic nitride such as gallium nitride.
If desired, the support layer may be a bulk layer and may be produced for example from silicon, gallium nitride, silicon carbide, aluminum nitride, or sapphire.
Another aspect is aimed at producing donor wafers with reduced losses, and therefore with more profitable use of the starting material (in this case single-crystal SiC). A process may be provided for producing a donor wafer intended to be used in a process for transferring successive thin layers of a given semiconductor material from the donor wafer to a receiving wafer. The process may involve producing a bulk slice of the semiconductor material and assembling the bulk slice and a support in order to form the donor wafer. These techniques may alleviate some of the drawbacks in conventional technology that exists when a slice from an ingot of a semiconductor material (e.g., SiC) is used as a bulk donor wafer.
If desired, the bulk slice may be produced by sawing an ingot or by thick-film epitaxy on a seed layer. If thick film epitaxy is used, the step of removing the seed layer may be implemented.
The bulk slice may be polished only on its face that is intended to come into contact with the support. In the prior art, both faces are typically coarsely polished. Polishing may be performed to a defined degree on the face of the bulk slice and the face of the support which are intended to come into contact with each other.
Assembling the bulk slice and the support may be carried out at a temperature and for a time such that wafer bonding or welding may be achieved between the bulk slice and the support. The semiconductor material of the bulk slice may be a single-crystal semiconductor and the support may be chosen from the group comprising the same semiconductor as the bulk slice but with a single crystal of inferior quality, the same semiconductor in polycrystalline form, or the same semiconductor as a different polytype. The bulk slice, the support, or both may be silicon, silicon carbide, or large-gap monometallic or polymetallic nitrides (e.g., gallium nitride). If desired, the support layer, the bulk slice, or both may be produced from silicon, gallium nitride, silicon carbide, aluminum nitride, or sapphire. If desired, other materials may be contemplated.
Further features, objects and advantages of the present invention will become more clearly apparent on reading the following detailed description of preferred embodiments of implementation of the invention, the description being given by way of non-limiting example and with reference to the appended drawings.